ORIGINAL: SeamusG

I use 6x32 hex head machine screws to bolt my engine to the mount. If the mount has flat rails top and bottom I will drill the holes in the mount large enough to allow the screw to pass through the mount (snug but not tight) and then use a nylon locking nut to secure it on the back side. I will also use a 6x32 tap and thread the hole through the mount. I do this when I do not have easy access to the back side of the mount for a wrench or nut driver. If the mount has a tapered bottom and a nut will nut tighten flush against the bottom of the mount I will again use the 6x32 tap. Typically a screw either 3/4'' or 1'' will be sufficient. Error on the long side of course.

HTH,